Viable planar pentacoordinate carbon (ppC) systems with a ppC bonded to a transition metal and embedded in a metallocene framework are reported. Our detailed global minima search shows that CAl₄MX₂ (M = Zr and Hf; X = F–I and C₅H₅) clusters with ppCs are appropriate candidates for experimental realization in the gas phase. The fulfillment of the 18 electron rule and electron delocalization is found to be crucial for the stabilization of these ppC arrangements.

The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level ab initio computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa₅, Li₅Mg⁺, Na₅Mg⁺, K₅Ca⁺, CaRb₅ ⁺, Rb₅Sr⁺, and SrCs₅ ⁺) adopts a singlet D_5h structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity.

Herein we introduce a strategy employing lithium atoms as a scaffold to stabilize an embryo for boron tubes. The systematical exploration of the potential energy surface via evolutionary algorithms allowed us to find that Li₂B₂₄ adopts a tubular structure formed by three stacked rings of eight borons each with two lithium atoms capping the tube. The lithium atoms are essential for stabilization because of the strong electrostatic interaction between the Li cations and the boron framework, and concomitantly, they compensate for the energy cost of distorting a quasi-planar or double ring B₂₄ cluster.

A single isoelectronic substitution by a smaller sized element, Be, is enough to stabilize a CGa ₅ ⁺ cluster with a planar pentacoordinate carbon at the centre.

Ar to Rn atoms formed exceptionally strong bonds with B ₃ ⁺ , where the Ng (HOMO) → B ₃ Ng ₂ ⁺ (LUMO) σ-donation is the key term to stabilize the complexes.